> LINETJS. 273 



defined. Many anatomical differences are found in the 

 family, amongst which may more particularly be men- 

 tioned the following : — 



(a) A diagonal muscle layer, neurochord cells, eyes, and 



frontal organ may be either present or absent. 



(b) A well-marked cephalic vascular head loop may be 



present, or the cephalic vessels may form an 

 anastomosing network. 



(c) The excretory system shews great variations in its 



backward extent ; the position of the tubules 

 may be dorsal or ventral, or both ; they may 

 reach forward to the cerebral organ, or may com- 

 mence some way behind it. Also some species 

 possess a number of ducts whilst only one pair is 

 present in others. 

 In the majority of the Lineidse, and indeed in manv 

 British forms, we are as yet in ignorance with regard to 

 many of these points, and until they have been deter- 

 mined it is useless to attempt to place the classification of 

 the family upon a more satisfactory basis. 



